1. Field of the Invention
The present invention relates to a torque sensor used in a power steering system of a vehicle, and more particularly to a torque sensor for a vehicle in which a detection coil assembly included in the torque sensor has a modified structure to be fixed using a sensor housing, without using a separate fixing member, thereby being capable of reducing the number of elements in the torque sensor.
2. Description of the Related Art
Generally, a vehicle can steer a running direction thereof in accordance with operation of a steering wheel connected to wheels via a steering shaft. However, when the friction between the wheels and the ground is high, it is impossible to easily and rapidly steer the vehicle because an increased force is required to operate the steering wheel. In order to solve this problem, a power steering device is used. The power steering device is installed, as an auxiliary power generating device, in a steering system to perform a servo operation to reduce the operating force for the steering wheel.
The steering shaft of a vehicle, in which a power steering device is equipped, is divided into an input shaft and an output shaft. The input shaft is coupled to a steering wheel, whereas the output shaft is coupled to a pinion engaged with a rack bar formed at a tie rod. The input and output shafts are connected by a torsion bar. When the steering wheel is rotated, the output shaft is rotated, so that the direction of wheels coupled to the tie rod is varied in accordance with a gearing operation carried out between the pinion and the rack bar. Where the friction between the wheels and the ground is high, the gearing operation between the pinion and the rack bar cannot be smoothly carried out. In this case, the output coupled to the pinion is rotated in a range less than the rotation range of the input shaft coupled to the steering wheel, so that the torsion bar is twisted. The twisting range of the torsion bar is transmitted in the form of an electrical signal to an electronic control unit. Based on the transmitted electrical signal, the electronic control unit controls the auxiliary power generating device coupled to the rack bar to compensate for rotation of the output shaft, and thus, to enhance the operating force of the steering wheel. The twist of the torsion bar is sensed by a torque sensor.
FIG. 1 is a sectional view illustrating the structure of a conventional torque sensor used in vehicles. As shown in FIG. 1, the conventional torque sensor includes a torsion bar 3 to coaxially connect a lower end of an input shaft 1 and an upper end of an output shaft 2, three detection rings 4a, 4b and 4c made of a magnetic material and fixedly fitted around connecting portions of the input and output shafts 1 and 2 in a state of being uniformly axially spaced apart from one another, and a cylindrical sensor housing 5 to enclose the detection rings 4a, 4b and 4c. 
The output shaft 2 is coupled, at a lower end thereof, with a pinion 2a engaged with a rack bar (not shown) formed at a tie rod (not shown) connected to wheels (not shown). The input shaft 1 is coupled, at an upper end thereof, with a steering wheel (not shown). Accordingly, when the steering wheel is rotated, the torsion bar 3, which connects the input and output shafts 1 and 2, is twisted, thereby causing the output shaft 2 to rotate.
The first and second detection rings 4a and 4b are fixedly fitted around the input shaft 1 to rotate through the same angle as the steering wheel (not shown), whereas the third detection ring 4c is fixedly fitted around the output shaft 2 to rotate together with the output shaft 2. Teeth 4d, 4e and 4f are formed at lower surfaces of the first and second detection rings 4a and 4b, and an upper surface of the third detection ring 4c. The teeth 4d, 4e and 4f are formed in the form of repeated protrusions and grooves.
A temperature compensating detection coil assembly 6 and a magnetically-resistant detection coil assembly 7 are disposed in the sensor housing 5 such that the detection coil assemblies 6 and 7 enclose the teeth 4d, 4e and 4f of the detection rings 4a, 4b and 4c. Wound coils 6a and 7a are included in the detection coil assemblies 6 and 7 to co-operate the teeth 4d, 4e and 4f, and thus, to form magnetic circuits, respectively. The temperature compensating detection coil assembly 6 is arranged to enclose facing portions of the first and second detection rings 4a and 4b, whereas the magnetically-resistant detection coil assembly 7 is arranged to enclose facing portions of the second and third detection rings 4b and 4c. A spacer 8 is interposed between the detection coil assemblies 6 and 7.
The facing area of the teeth 4e and 4f of the second and third detection rings 4b and 4c is varied in accordance with twist of the torsion bar 3. Due to the facing area variation, the inductance of the magnetically-resistant detection coil assembly 7 varies. The rotation deviation between the input and output shafts 1 and 2 is detected by measuring the inductance variation.
In order to accurately detect a torque applied to the torsion bar 3, the detection coil assemblies 6 and 7 must be fixed in the sensor housing 5 so that the positions of the detection coil assemblies 6 and 7 are not shifted. Accordingly, a “C”-shaped fixing ring 9a is fixedly mounted in magnetically-resistant detection coil assembly 7 to fix the positions of the detection coil assemblies 6 and 7. Also, a wave washer 9b is interposed between the fixing ring 9a and the magnetically-resistant detection coil assembly 7. By the fixing ring 9a and wave washer 9b, the axial positions of the detection coil assemblies 6 and 7 in the sensor housing 5 are fixed.
However, since separate fixing members such as the fixing ring 9a and wave washer 9b must be used to fix the detection coil assemblies 6 and 7, the conventional torque sensor involves an increase in the number of constituent elements, and thus, a degradation in productivity.